The present invention relates to data storage devices and, in particular, to servo patterns on magnetic media within data storage devices.
In a magnetic disc drive, data is stored on one or more discs, which are coated with a magnetic medium. The magnetic medium is typically divided into a plurality of parallel data tracks, which are arranged concentrically with one another perpendicular to the disc radius.
The data is stored and retrieved by a transducer or xe2x80x9cheadxe2x80x9d that is positioned over a desired track by an actuator arm. The actuator arm moves the head in a radial direction across the data tracks under control of a closed-loop servo system based on position information or xe2x80x9cservo dataxe2x80x9d, which is stored within dedicated servo fields. The servo fields can be interleaved with data sectors on the disc surface or can be located on a separate disc surface that is dedicated to storing servo information. As the head passes over the servo fields, it generates a read back signal that identifies the location of the head relative to the center line of the desired track. Based on this location, the servo system moves the actuator arm to adjust the head""s position so that it moves toward a desired position.
The servo field patterns are typically written onto the disc surface through the product read/write head after the disc has been assembled within the disc drive housing to form a head disc assembly (HDA). A machine called a Servo Track Writer (STW) clamps the HDA along the X, Y and Z axes and then through some method, measures the position of the read/write head and positions the head to the appropriate radial locations to write the servo tracks. Typical methods of obtaining position feedback include the use of a laser interferometer or an optical encoder.
The servo track writer also provides a clock signal with which to align adjacent servo tracks in time. Typically, a clock head is inserted into the HDA and is flown on one of the disc surfaces. The clock head is used to write a clock signal onto the disc surface. This clock signal is then used to run a phased locked loop (PLL) for obtaining a stable reference signal with which to write adjacent radial tracks in a servo pattern with sufficient radial coherence.
In self-servo track writer systems, the servo tracks are written by bootstrapping the position of each track by the position of the previous track in the HDA with no external references. In these systems, errors in the placement of the servo tracks tend to propagate radially from one track to the next as each track is written in the HDA.
As the storage density of disc drives continues to increase, thermal decay of the individual magnetic domains in the servo fields is becoming a greater concern. This thermal decay is due to a xe2x80x9csuperparamagneticxe2x80x9d effect in which individual grains of the magnetic medium within a magnetic domain can spontaneously reverse polarity This causes a degradation in the quality of the read back signal that is produced by the magnetic domain. As the quality of the servo patterns decays, it becomes more difficult for the closed-loop servo control system to generate accurately the reference position signals with which the disc drive finds user data on the disc surface. As a result, some user data can be lost.
One approach that has been proposed to reduce the effects of thermal decay is the use of hard servo patterned media. Hard servo patterned media refers to producing physical features in or on the recording disc that cause the read back transducer to produce a signal that can be used to generate a position error signal (PES) with respect to the center line of the desired data track. Multiple approaches to fabricating servo patterned media have been proposed, which produce multiple classes of servo patterned media.
There are two main methods of producing hard servo patterned media. In the first method, the magnetic disc is fabricated using currently known methods, minus the carbon overcoat and lube. Then, portions of the magnetic medium are etched away to define the servo patterns and other features. The etched magnetic medium is then protected by a layer of carbon, and the lube is applied in the final fabrication step. The pattern of transitions between magnetic and non-magnetic media are used to define the servo patterns. This type of patterning technique may be less susceptible to the superparamagnetic effect if one dibit is formed from one etched island such that the magnetic material of adjacent dibits are physically isolated from one another. This may limit magnetic interaction between the grains of adjacent di-bits.
In the second method, the substrate surface is patterned to form a topography that defines the desired servo pattern features. The magnetic medium is then deposited on top of the featured substrate. Spacing loss between the head and the medium surface due to the substrate topography produces a read back signal that can be used to generate a position error signal (PES). However, because of the continuous magnetic medium film between the raised islands and recessed valleys along the topography, it is not currently clear that thermal decay will not occur if the raised islands are used to define the servo dibits.
The present invention addresses these and other problems, and offers other advantages over the prior art.
One aspect of the present invention is directed to a method of refreshing magnetization of a servo pattern in a magnetic medium within a disc drive, wherein the servo pattern is at least partially defined by physical features of the medium. The method includes reading servo information through a transducer in the disc drive from a first portion of the servo pattern. The transducer is positioned at a radial position relative to the medium based on the servo information. The magnetization of a second portion of the servo pattern is then refreshed at the radial position with a magnetic field generated by the transducer.
Another aspect of the present invention relates to a method of maintaining magnetization of a servo pattern in a disc drive. The method includes operating the disc drive in a normal operating mode in which user data areas within the magnetic medium are accessed by reading servo information from a first portion of the servo pattern through a transducer in the disc drive and positioning the transducer at a desired radial position relative to the medium based on the servo information. The disc drive is then operated in a refresh mode in which the first portion of the servo pattern is re-magnetized with a magnetic field generated by the transducer. The disc drive is returned to the normal operating mode after operating in the refresh mode.
In one embodiment, a representation of a magnetic field strength produced by the magnetic medium within the first portion of the servo pattern is measured while in the normal operating mode. If the representation of the magnetic field strength is less than a threshold level, the disc drive is switched from the normal operating mode to the refresh mode.
Yet another aspect of the present invention is directed to a disc drive, which includes a data storage disc and a transducer. The data storage disc includes a magnetic medium having a servo area and a user data area. The servo area is defined by a set of magnetic domains having a magnetization pattern, wherein at least some of the magnetic domains are defined by physical features of the medium. The transducer is adapted to generate a read signal in response to magnetic transitions within the servo area and the user data area and is adapted to generate a write signal in the form of a magnetic field. The disc drive further includes an apparatus for refreshing the magnetization pattern of the set of magnetic domains with the magnetic field generated by the transducer.